Due to rapid increase of the volume of data communications, optical modules compatible with 400 Gigabit Ethernet (400 GbE) standards are being developed. One of the 400 GbE standards adopts wavelength division multiplexing (WDM) with 4-level pulse amplitude modulation (PAM4) to increase the bit rate. The current target of 400 GbE transmission distance is basically a short distance such as 2 km or 10 km; however, discussions on standardization of long-distance optical communications over 10 km (e.g., 40-km transmission) are about to start. In increasing the transmission distance for 400 GbE, a scheme of securing a dynamic range using an optical amplifier may be discussed, as in ER4 which is a standard for 40 Km transmission of 100-Gbps Ethernet.
FIG. 1 is a schematic diagram of a WDM receiver frontend circuit. A variable optical attenuator (VOA) and a semiconductor optical amplifier (SOA) are provided in the receiver. When a low-power WDM signal is received, the received signal in which multiple channels with different wavelengths are multiplexed is amplified collectively by the SOA. When a high-power WDM signal is received, the WDM signal is attenuated collectively by the VOA. After demultiplexing, the respective wavelengths are detected at the associated photodetectors such as photo diodes (PDs) to secure the dynamic range.
In WDM systems, optical filters are generally used for demultiplexing. As illustrated in the states (A) and (B) of FIG. 1, when amplifying the received signal light collectively using the SOA, amplified spontaneous emission (ASE) increases, and consequently, the photodetection sensitivity decreases as illustrated in the state (C).
Degradation in photodetection sensitivity due to optical noise becomes more serious when the number of levels of multilevel modulation increases. In case of multilevel modulation at or over 4 levels, the signal-to-noise ratio becomes lower than that of binary modulation, and the influence of ASE noise on the photodetection sensitivity becomes conspicuous. For example, an optical demultiplexer or a wavelength filter for LAN-WDM has a bandwidth of about 6 nm to 7 nm in order to cover the wavelength range of input light. With such a broad passband, ASE noise cannot be reduced sufficiently (see the State (C) of FIG. 1). In coarse-WDM (CWDM), the wavelength interval of transmitted signal light is broader, and the bandwidth of the optical demultiplexer may become as wide as about 30 nm. The ASE noise reduction becomes more difficult.
A configuration that adaptively suppresses leakage of ASE into the photodetectors has been proposed. See, for example, Patent Document 1.                Patent Document 1: Japanese Patent Application Laid-open Publication No. 2005-210264.        